This invention relates a low expansion transparent crystallized glass-ceramic suitable for use as various materials for which low thermal expansion characteristics and transparency are strictly required such, for example, as materials for optical parts, materials for large type telescope mirror blanks, materials for a ring laser gyroscope, materials for precision parts such as parts of a reference gauge for precision measurement and various electronic materials.
Known in the art are low expansion transparent crystallized glass-ceramics formed by melting and subjecting to heat treatment a base glass of a SiO.sub.2 -Al.sub.2 O.sub.3 - Li.sub.2 O system containing a nucleating agent. These crystallized glass-ceramics generally contain beta-quartz solid solution as a main crystalline phase and have a coefficient of thermal expansion of (.alpha..times.10.sup.-7 /.degree. C.) within the range of from -10 to +10.
Recent tendencies in the industry are that higher and more strict characteristics as described below are required for the materials used for the above described purposes:
(1) The glass-ceramic is uniform and stable and has little variation in the coefficient of thermal expansion in spite of variation in heat treatment conditions. PA1 (2) The base glass can be molten and refined easily so that the glass-ceramic substantially has little or no cord, foam or inclusion and has an optical homogeniety of a high degree. PA1 (3) The crystal produced is fine and the glass-ceramic has an excellent transparency, particularly an excellent light transmissivity in the visible ray region. PA1 (4) The glass-ceramic contains substantially little or no Na.sub.2 O and K.sub.2 O ingredients, because if the glass-ceramic contains Na.sub.2 O and K.sub.2 O ingredients, ions of these ingredients will diffuse in the course of processing of the glass-ceramic and thereby adversely affect properties of the glass-ceramic with resulting occurrence of troubles. Besides, these ingredients will increase the value of the coefficient of thermal expansion to an undesirable degree.
As one of the above described prior art low expansion transparent crystallized glass-ceramic, the U. S. Pat. No. 3,499,773 discloses a low expansion transparent crystallized glass-ceramic product selectively imparting an opaque portion obtained from a base glass SiO.sub.2 -Al.sub.2 O.sub.3 -Li.sub.2 O system containing one or more of TiO.sub.2, ZrO.sub.2 and SnO.sub.2 as a nucleating agent and optionally containing a small amount of other ingredients such as CaO, MgO and BaO. The U. S. Pat. No. 4,285,728 discloses crystallized glass-ceramics for telescope mirror blanks obtained from a base glass of a SiO.sub.2 -Al.sub.2 O.sub.3 -Li.sub.2 O-(CaO+MgO+ZnO+Na.sub.2 O) system containing TiO.sub.2 and ZrO.sub.2 as nucleating agents. The U. S. Pat. No. 4,851,372 discloses a transparent crystallized glass-ceramic for telescope mirror blanks obtained from a basic glass of SiO.sub.2 -Al.sub.2 O.sub.3 -Li.sub.2 O-ZnO-BaO system containing TiO.sub.2 and ZrO.sub.2 as nucleating agents in which the value of .alpha. becomes substantially zero in the temperature range between 0.degree. C. and 50.degree. C.
The specifications of these publications, however, do not particularly describe the specific problems to be solved by the present invention, particularly the recent requirements of the industry (1), (2) and (3) described above. In fact, these prior art crystallized glass-ceramics are disadvantageous in that variation of the coefficient of thermal expansion accompanied by variation of heat treatment conditions is large and also that high temperatures of 1500.degree. C. and over are required for melting the base glasses and hence it is difficult to manufacture products having an excellent optical homogeniety.
The U.S. Pat. No. 3,380,818 discloses a low expansion transparent crystallized glass-ceramic having a liquidus temperature below 2460.degree. F. and formability which is suitable for large-scale production and obtained from a base glass of a SiO.sub.2 -Al.sub.2 O.sub.3 -Li.sub.2 O-MgO system containing TiO.sub.2 and ZrO.sub.2 as nucleating agents. The U. S. Pat. No. 4,018,612 discloses a transparent crystallized glass-ceramic used for cooking purposes having an excellent anticorrosion property to detergents and an excellent infrared transmissivity and obtained from a base glass of a SiO.sub.2 -Al.sub.2 O.sub.3 -Li.sub.2 O-MgO-ZnO system containing TiO.sub.2 and ZrO.sub.2 as nucleating agents. The U.S. Pat. No. 4,707,458 discloses a transparent crystallized glass-ceramic for a ring laser gyroscope having a coefficient of thermal expansion which is substantially zero and obtained from a base glass of a SiO.sub.2 -Al.sub.2 O.sub.3 -Li.sub.2 O-MgO-ZnO system containing TiO.sub.2 and ZrO.sub.2 as nucleating agents. Further, European Patent Application No. EP-A-0437228 discloses a crystallized glass-ceramic suitable for use as a cooktop sheet which exhibits substantially little or no distortion even by a fast heat treatment for efficient production and obtained from a base glass of a SiO.sub.2 - Al.sub.2 O.sub.3 -Li.sub.2 O-MgO-ZnO-Bao and/or SrO system containing TiO.sub.2 and ZrO.sub.2 as nucleating agents.
These crystallized glass-ceramics, however, do not suggest any improvement about the required properties described in (1), (2) and (3) above. In fact, these crystallized glasses are disadvantageous in that the coefficient of thermal expansion tends to vary largely with variation in heat treatment conditions and hence it is difficult to impart a uniform coefficient of thermal expansion to respective parts of the products made from the glass-ceramics and also to respective manufacturing lots of the glass-ceramics. Moreover, high temperatures of about 1550.degree. C. and over are required for melting and refining the base glasses and this makes it difficult to manufacture products having an excellent optical homegeniety in a large scale.
It is, therefore, an object of the present invention to provide a low expansion transparent crystallized glass-ceramic with improved properties, namely, having reduced variation in the coefficient of thermal expansion (.alpha.) accompanied by variation in conditions for heat treatment for crystallization and having a more excellent optical homogeniety owing to improved melting and refining properties of the base glass.